Categorized | Climate Change

Reduce Soot, Save Polar Bears

Black carbon is an accomplice in the recent increase in global ice melt, affecting everything from polar bear habitat in the Arctic, to glacial fed drinking water in the Himalayas.  In a recent study, Menon at al. (2010) found that “Most of the change in snow and ice cover – about 90 percent – is from aerosols. Black carbon alone contributes at least 30 percent of this sum.”

Black carbon is probably the third largest atmosphere warming agent, after carbon dioxide and methane (on the basis of radiative forcing).  Black carbon has a strong warming effect because it absorbs visible light and transfers the energy to the atmosphere, warming it.  Due to its short atmospheric lifetime, black carbon does the most damage near the areas where it is emitted, especially in terms of atmospheric warming and health issues due to inhalation.  Black carbon settles quickly from the atmosphere, and when it lands on snow and ice, the darkened snow and ice absorbs more heat and melts more quickly.  For this reason, black carbon has been linked to the melting of Arctic ice and Himalayan glaciers, especially because most emissions occur in China and India, and above 40⁰N, where they are likely to be transported to the Arctic (see Princeton University Report).

Black carbon is an aerosol produced during poor combustion of carbon-based fuels (as opposed to carbon dioxide, which is produced in all circumstances), and together with organic carbon is one the major components in soot.  Sources of black carbon include diesel engines in various types of vehicles, furnaces, cook stoves, and forest fires, as well as some industrial processes.  Between 25% and 35% of atmospheric black carbon comes from China and India, due to combustion of wood, coal, and other fuels for household uses.  Europe, North America and eastern European countries emit about 13% of all black carbon, mostly from contained combustion.

Unlike carbon dioxide and other greenhouse gases, there are many uncertainties about the exact contribution of black carbon to global warming.  According to the Princeton University Report, this is in part due to the fact that black carbon is emitted with different amounts of various other aerosols and gases during different types and conditions of combustion.  These accompanying emissions, depending on their concentrations (which vary with the type of combustion), can minimize or increase the black carbon radiative forcing the common energy measure used in climate studies.  (That is why contained combustion contributes more to warming than open burning: because it releases less of the other types of emissions, including organic carbon, which mitigates the warming effect of black carbon.)  Also, while carbon dioxide can stay in the atmosphere for 100 years, black carbon has a short atmospheric lifetime, making its effects more localized.  Because of these and other characteristics, black carbon has not been formally addressed as a warming agent, or a possible target for decreasing global warming in the United Nations Framework Convention on Climate Change.

Various authors and policy makers advocate targeting black carbon as a more effective warming reduction tool in the near term, without ever losing sight of the fact that, in the long run, reducing greenhouse gases is the only way to reduce global warming.  The arguments for black carbon reduction are reinforced by the fact that technologies to reduce black carbon emissions already exist, such as retrofitting diesel vehicles with filters or switching from diesel to natural gas, and replacing inefficient cook stoves with cleaner burning ones.  More importantly, reducing black carbon emissions will also improve the quality of life and health of the communities most impacted by it, especially in developing countries.

As early as 2001, Jacobson found through simulations that any reduction of black carbon emissions and associated organic matter might slow global warming more than emission reduction of CO2 or CH4 for a specific period.  Ramanathan & Carmichael (2010) state that “If the immediate target for [warming] control shifts entirely to black carbon (owing to its health impacts) without a reduction in non-black carbon aerosols, the elimination of the positive forcing by black carbon will decrease both the global warming and the retreat of sea ice and glaciers.”  Michael MacCracken, from the Climate Institute in Washington, DC, in a recent brief advocated for a reduction in short-lived emissions, including black carbon, as a way to tackle short-term reduction in atmospheric warming.  As recently as June 14, the United Nations released a report which states that cutting black carbon and methane emissions would slow the rate of warming up until about 2040, and also includes a list of nine measures that can be undertaken immediately to reduce black carbon emissions.  Black carbon will be addressed at the UNFCCC COP 17 in Durban this coming November/December.

However, as mentioned before, there are several other variables to be taken into account, and a 2005 study by Bond & Sun concluded that, while the efforts to reduce black carbon emissions are for the most part viable, such reductions cannot counteract global warming for reasons ranging from black carbon behavior in the atmosphere, to difficulties in implementing improvements in developing countries, to costs, especially for developed countries already committed to CO2 reductions.  In spite of all the challenges, the authors note that reducing black carbon emissions has several advantages, including “providing a mechanism for all nations to participate in global stewardship with a more realistic definition of global change.”  Sounds like a worthy undertaking

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- who has written 12 posts on dotWild.

Astrid Caldas is the Climate Change & Wildlife Science Fellow for Defenders of Wildlife. Astrid provides scientific support for Defenders, including providing technical assistance for integrating climate adaptation into programs, doing synthetic research, and publishing papers and reports on climate and wildlife issues.

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dotWild is the blog of scientists and policy experts at Defenders of Wildlife, a national, nonprofit membership organization dedicated to the protection of all native animals and plants in their natural communities.